The Order of St. Patrick was established in 1783 as the senior order of chivalry in the Kingdom of Ireland. The colour of its honours needed to differ from those of the Order of the Garter (dark blue) and the Order of the Thistle (green). Orange was considered, but felt to be too sectarian, so the lighter blue of the Irish arms was chosen.[6] Knights and officers of the order wore a "sky blue" mantle and riband, a hat lined with "blue", and a badge ringed with "blue" enamel.[7] The name "St. Patrick's blue" was common but never officially used by the Order.[1][8] The exact shade of blue used varied over time. A sky blue tinged with green was used by Lord Iveagh in 1895 and confirmed in 1903.[1]

There has been debate over the extent to which blue was a national colour of Ireland prior to the creation of the Order, and whether it was associated with Saint Patrick himself independently of the Order. Shane Leslie speculated that the green-blue of St Patrick's blue might be "but a reminiscence of the woad-stain used by all colour-loving Celts".[9]Constance Markievicz believed blue was "the old colour of Ireland" and incorporated it in the regalia of the Irish Citizen Army (ICA).[10] The ICA banner, the Starry Plough, has a blue field. Antiquarian nationalist Francis Joseph Bigger considered St. Patrick's blue a "fake colour" and Saint Patrick's Flag a "fake flag".[11] More recently, Peter Alter[12] and Christina Mahony[13] have supported the historicity of the colour, while Brian Ó Cuív questioned it.[3] The Irish College in Paris, completed in 1776,[14] was renovated in 2002; the paint uncovered on the chapel walls was described as "St Patrick's blue" by a visiting journalist.[15] As regards green in association with Patrick: in 1681, Thomas Dineley reported people wearing crosses of green ribbon in their hats on Saint Patrick's Day.[16]

In the 1930s, the Army Comrades Association's Saint Patrick's blue shirts earned it the nickname of Blueshirts. It was a quasi-Fascistshirted movement which rejected green as associated with its republican opponents.[24] The saltire flag of the Blueshirts was a variant of Saint Patrick's Flag with the white background replaced with a blue background. W. T. Cosgrave described the colour as "in perfect, traditional, national accord with our history and in close association with the most revered and venerated memory of our patron Saint".[25]

The Irish Army Band's first uniform was St Patrick's blue, but this was soon changed to navy.[26] The Mounted Escort ceremonial cavalry of 1932–48 were nicknamed "Blue Hussars" from their uniforms, whose colour was sometimes described as St. Patrick's blue.[27][28][29] The uniform introduced in 1970 for Aer Lingusair hostesses and ground crew[30] combined green and St Patrick's Blue, described in The Irish Times as "a sparkling new colour".[31] The 1970 uniform was replaced in 1975, after a design consultancy developed a common corporate image with a colour scheme of dark bottle green, bright green, and "a strong blue".[32]

^Stewart, Georgiana L. (14 August 1893). "Protest To The Queen From Irish Women Against Home Rule.". The Times (34029). p. 6; col E. The whole was contained in a very handsome walnut casket lined with Irish poplin of the shade known as St. Patrick's blue, which is the colour of the riband worn on the robes of the Knights of St. Patrick.

^"Rules of Racing and Irish National Hunt Steeplechasing Rules". The Curragh, Co. Kildare: Irish Turf Club. 12 February 2009. p. 57; §105. Retrieved 4 March 2010. Horses the property of the Irish National Stud Co. Ltd. may be raced under these Rules in the name of the President of Ireland, who shall be deemed to be the lessee of such horses.

^O’ Reilly, Chryss Goulandris. "Chairman's Statement 2008". Reports and Financial Statements for the year ended 31 December 2008. Irish National Stud Co. Ltd. p. 5. Retrieved 4 March 2010. In addition the Irish National Stud owned Suailce. Racing in the colours of H E the President and trained by our director Dermot Weld, she was a high class winner here at home.